The algorithm as described in the blogpost is probably not implemented as a straightforward piece of procedural code that goes step by step through the input flightplan waypoints as described. It may be implemented in a way that incorporates some abstractions that obscured the fact that this was an input error.

If from the code’s point of view it looked instead like a sanity failure in the underlying navigation waypoint database, aborting processing of flight plans makes a lot more sense.

Imagine the code is asking some repository of waypoints and routes ‘find me the waypoint where this route leaves UK airspace’; then it asks to find the route segment that incorporates that waypoint; then it asserts that that segment passes through UK airspace… if that assertion fails, that doesn’t look immediately like a problem with the flight plan but rather with the invariant assumptions built into the route data.

And of course in a sense it is potentially a fatal bug because this issue demonstrates that the assumptions the algorithm is making about the data are wrong and it is potentially capable of returning incorrect answers.